Preparation of single crystal films of lithium niobate by radio frequency sputtering

ABSTRACT

A METHOD OF RADIO FREQUENCY SPUTTERING LITHIUM NIOBATE THIN, SINGLE CRYSTAL FILMS HAVING BULK PROPERTIES IS DISCLOSED. THE SPUTTERING PROCESS IS PREFERABLY CARRIED OUT AT SUBSTRATE TEMPERATURES IN A RANGE OF 450*C. TO 870*C. THE SUBSTRATE IS PREFERABLY SINGLE CRYSTAL C OR A-A-AL2O3 OR MAY BE A SINGLE CRYSTAL OF GROWN LITHIUM NIOBATE. RADIO FREQUENCY POWERS IN THE RANGE OF 25 TO 150 WATTS FOR A FOUR-INCH DIAMETER CATHODE AND DEPOSITION RATES OF LESS THAN 0.4 A. PER SECOND MAY BE UTILIZED. ARGON PRESSURES DURING SPUTTERING ARE IN THE RANGE OF 25 MICRONS TO 44 MICRONS. THIN, SINGLE CRYSTAL FILMS UP TO 10,000 A. IN THICKNESS HAVE BEEN PRODUCED UTILIZING THE ABOVE SPUTTERING TECHNIQUE. THE RESULTING FILMS HAVE PROPERTIES SIMILAR TO CRYSTALS GROWN IN BULK.

March 14, 1972 v SADAGOPAN 3,649,501

PREPARATION OF SINGLE CRYSTAL FILMS OF LITHIUM NIOBATE BY RADIO FREQUENCY SPUTTERING Filed June 18, 1970 2 Sheets-Sheet 1 I-INTRODUCING A SINGLE CRYSTAL SUBSTRATE INTO R.F.

SPUTTERING APPARATUS.

Z-POSITIONING A HOT PRESSED CATHODE OF LITHIUM NIOBATE PARALLEL TO AND SPACED FROM SAID SUBSTRATE IN SAID R.F. SPUTTERING APPARATUS.

S-EVACUATING THE SPUTTERING APPARATUS TO A PRESSURE OF APPROXIMATELY 2X10" TORR. AND GETTERING UNDESIRED GASES FROM THE APPARATUS.

4-INTRODUCING ARGON INTO THE SPUTTERING APPAR- ATUS AT A PRESSURE IN THE RANGE OF 25 TO 44 MICRONS.

S-PRE-SPUTTERING TO INSURE THE CLEANLINESS OF THE CATHODE WITH THE SHUTTER SHIELD- ING THE SUBSTRATE.

S-APPLYINS R.F. POWER TO THE LITHIUM NIOBATE CATHODE SUFFICIENT TO PROVIDE A POWER DENSITY OF 2-I2.5wATTS/IN FOR A TIME SUFFICIENT TO DEPOSIT A SINGLE cRYSTAL FILM ON A SUBSTRATE HEATED TO A TEMPERATURE IN THE RANGE OF 450C- 870C. DEPOSITION RATES ARE 0.4A/SEO OR LESS.

FIG. I

INVENTOR VARADACHARI SADAGOPAN A RNEY March 14, 1972 v SADAGQPAN 3,649,501

PREPARATION OF SINGLE CRYSTAL FlLMS OE LITHIUM NIOBATE BY RADIO FREQUENCY SPUTTERING Filed June 18, 1970 2 Sheets-Sheet 2 FIG. 2

United States Patent 3,649,501 PREPARATION OF SINGLE CRYSTAL FILMS 0F LITHIUM NIOBATE BY RADIO FREQUENCY SPUTTERING Varadachari Sadagopan, Ossining, N.Y., assignor to International Business Machines Corporation, Armonk,

Filed June 18, 1970, Ser. No. 47,483 Int. Cl. C23c 15/00 U.S. Cl. 204--192 17 Claims ABSTRACT OF THE DISCLOSURE A method of radio frequency sputtering lithium niobate thin, single crystal films having bulk properties is disclosed. The sputtering process is preferably carried out at substrate temperatures in a range of 450 C. to 870 C. The substrate is preferably single crystal c or a-a-Al O or may be a single crystal of grown lithium niobate. Radio frequency powers in the range of 25 to 150 watts for a four-inch diameter cathode and deposition rates of less than 0.4 A. per second may be utilized. Argon pressures during sputtering are in the range of 25 microns to 44 microns. Thin, single crystal films up to 10,000 A. in thickness have been produced utilizing the above sputtering technique. The resulting films have properties similar to crystals grown in bulk.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to an RF sputtering process for producing thin, single crystal films of lithium niobate which possess, in particular, optical properties similar to that obtained in bulk single crystals.

DESCRIPTION OF THE PRIOR ART The deposition of sputtered lithium niobate films has been reported in the literature in the last few years. In the prior art technique disclosed, lithium niobate was deposited by triode sputtering in an argon-oxygen gas mixture containing 510% oxygen. Fused quartz or sapphire substrates covered with thin layers of chrome and gold were utilized. Using a lithium niobate cathode and substrate temperatures between 100 and 325 C., both polycrystalline and oriented films of lithium niobate were produced. Since then, the literature has been notably silent on any technique which would provide single crystal thin films of lithium niobate having properties similar to those of bulk lithium niobate.

Other recent literature discussing techniques for the deposition of films of piezoelectric materials has indicated that techniques such as vacuum deposition or vacuum evaporation have not produced thin films of lithium niobate with satisfactory characteristics. In all instances where thin films of lithium niobate have been produced by sputtering, an AC sputtering technique has been uti lized.

In AC sputtering, a low frequency potential, i.e., approximately 60 cycles/second is applied between the target and the anode and, as a result, there is small amount Patented Mar. 14, 1972 of sputtering from the anode in addition to the sputtering which occurs at the target electrode. In the usual case, the anode is the substrate upon which deposition of the target material is desired. In AC sputtering, the duty cycle and the applied potential are adjusted so that most of the sputtering is from the target resulting in a net deposition of target atoms on the anode (substrate).

RF sputtering techniques are well known in the prior art and are similar to AC sputtering systems except that the low frequency source is replaced by a source producing RF voltages at frequencies of about 13.56 mHz. between the target and the anode. RF sputtering is particularly applicable to the deposition of insulating materials since, by the application of RF potentials, a target insulator can be made negative with respect to a substrate anode and a glow discharge can be established between these electrodes. The present invention utilizes an RF sputtering method to deposit single crystal lithium niobate films and defines a specific range of conditions in which thin films of lithium niobate having properties similar to bulk crystals are produced.

SUMMARY OF THE INVENTION The method of the present invention, in its broadest aspect, comprises the step of depositing, by RF sputtering, lithium niobate on the surface of a single crystal substrate. The single crystal substrate must exhibit a favorable lattice match to reduce stress in the final film and to insure epitaxial growth of the sputtered lithium niobate. Also, the step of pretreating the cathode prior to deposition is utilized to prevent degradation of the cathode and consequent nonstoichiometry of the resulting epitaxial film.

In accordance with more particular aspects of the invention, a hot pressed lithium niobate cathode is presputtered for 15 to 30 minutes in the sputtering environment with the hot substrate shielded. .The cathode is then cooled for 15 minutes between the presputtering and sputtering steps. Cooling avoids over heating of-the cathode and consequent spalling and breakage of the cathode. In the next cycle, presputtering is carried out for one or two minutes, the shield removed and the sputtering continued. The substrate should be of single crystal material and may be a single crystal of grown lithium niobate or may be c or asapphire. The cathode is approximately 4 inches in diameter. Radio frequency powers in the range of 25 to 150 watts are then applied to the cathode to cause deposition of single crystal lithium niobate at deposition rates of 0.4 A. per second and less. Sputtering is carried out at argon pressures in the range of 25 microns to 44 microns. A magnetic field of 30 to gauss is applied during the sputtering operation. Using the above-described technique, a single crystal film up to 10,000 A. in thickness is produced. The resulting single crystal films have the lattice constant and optical transmission characteristics identical to that of single crystal, bulk LiNbO These properties permit the resulting films to be used as a high temperature transducer material in acoustical environments and as optical parametric oscillators, amplifiers and electro-optic modulators in the electro-optical regime.

Based on recent research in the area of micro wave acoustic surface waves, LiNbO has been shown promise for single processing devices that are 100,000X smaller and lighter than their electromagnetic counter parts. Some of the applications are as delay lines (for false radar return), pulse compressors and travelling wave amplifiers. The electro optic capability of LiNbO has been utilized so far in making light modulators which find application in mode-locked lasers of considerable importance for communication and optical radar applications. Other applications for large area LINbOg crystals have been described in literature. Thus, there is a need for large area, single crystal films in a variety of device applications.

It is, therefore, an object of this invention to provide single crystal films of lithium niobate required for acoustical and electro-optic applications.

Another object is to provide a method of radio frequency sputtering which produces single crystal films of lithium niobate which have properties comparable to lithium niobate in bulk. Still another object is to provide a method for epitaxially depositing stoichiometric thin films of single crystal lithium niobate.

Still another object is to provide single crystal lithium niobate films which are free from inclusions, striae with no gradation in composition across the film surface area or by depth.

Yet another object is to provide single crystal lithium niobate films having substantially larger surface area than heretofore obtainable.

The foregoing and other objects, features and advantages of the invention will become apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.l is a fiow chart diagrammatically outlining the principal method steps for the RF sputtering of single crystal lithium niobate films.

FIG. 2 is a schematic drawing of apparatus used to provide RF sputtered single crystal films of lithium niobate.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with preferred method steps as outlined in flow chart form in FIG. 1, a single crystal thin film of lithium niobate is produced as follows:

Step 1.-Introducing a single crystal substrate into the RF sputtering apparatus of FIG. 2.

In order to achieve growth of single crystal lithium niobate films, a single crystal substrate having the same or substantially the same lattice match is required. Accordingly, a single crystal of grown lithium niobate may be utilized. Other substrates which have been utilized successfully are a and c-sapphire substrates. These latter substrates are only slightly mismatched with respect to the lattice of the sputtered lithium niobate films and, as such, epitaxial growth can be achieved without the introduction of stresses when the films are utilized at room temperature. Substrate areas should not exceed the area of the cathode and are only limited in size by the availability of the appropriate substrate materials. Grown single crystals of bulk lithium niobate of .5 to 1 sq. inch are obtainable only with great difiicu'lty and bulk films (single or polycrystalline) having larger surface areas are rare, if they are at all obtainable. Where lithium niobate films of large surface area are required, it is, of course,

necessary to use single crystal sapphire substrates.

crystal films are also stoichiometric in nature and contain no impurities which would adversely affect their operation in either the acoustical or electro-optical environments. RF sputtering apparatus suitable for carrying out the present invention is schematically shown in FIG. 2. A detailed description of the apparatus and its method of operation are shown in a co-pending application titled Improved Thin Flms and the Sputtering Process for Producing Them in the name of J. J. Cuomo, A. F. Mayadas and R. Rosenberg, Ser. No. 837,738, filed June 30, 1969, and assigned to the same assignee as the present invention.

The apparatus of the above-identified co-pending application and that shown in FIG. 20f the present application are identical structurally with the exceptionthat the cathode of the present disclosure is made of lithium niobate. a

Referring now to FIG. 1, an RF source 10 provides'a voltage varying at radio frequency'rates between'a target 12 and a substrate 14. RF source 10 is electrically coupled to target 12 via lead 16 and capacitor 18 while substrate 14 is grounded. Target 12 is attached to an electrode 20 which is cooled by a flow of water through tubulation 21. Substrate 14 is maintained at a desired temperature by a heater 22. Cooling coils 24, which surround anode structure 26, remove excess heat from the anode region. When, for example, sputtering at room temperature is required.

Helmholtz coils 28 which are disposed circumferentially about electrode 20 and anode structure 26 provide a magnetic filed of approximately 30 to gau'ss in a direction perpendicular to target 12 and anode structure 26. The purpose of the magnetic filed is to improve the probability of an ionizing event in the sputtering environment so that the efiiciency of sputtering is increased. The magnetic filed also increases the induced bias on substrate A metal dome 30 encloses the above-described elements. Dome 30 has openings 32, 34, the former for admitting an ionizable gas and the latter for creating a vacuum within dome 30. Ionizable gas such as argon may be utilized although other gases, such as neon and kryton, may also be utilized. A shield 36 surrounds target 12, which prevents sputtering from portions of the apparatus other than from the actual target source 12 which is to be deposited as a film upon substrate 14. A shutter 38 is movably positioned between target 12 and substrate 14. Knob assembly 40 is utilized for moving shutter 38 from beneath target 12. Shutter 38 is normally positioned'beneath target 12 during presputtering but is moved aside during the actual deposition process.

Titanium getter-ion pump 42 is used to getter active gases such as oxyen, nitrogen, etc. from. within dome 30 before sputtering begins. Dome 30 isevacuated by. a vacuum pump (not shown) connected by tubing .43 at opening 34 to a pressure of approximately 2X10- torr. Titanium getter-ion pump 42 is then activated to getter oxygen or other undesired gases from the system.

Step 3.--Evacuating the sputtering apparatus to a pressure of approximately 2 l0 torr and gettering undesired gases from the apparatus. l

This step establishes the initial conditions which are required for a sputtering operation.

Step 4.Introducing argon into the sputtering apparatus at a pressure in the range of 25 to 44 microns.

The argon is provided as an ionizable gas to provide positive ions which, when accelerated in a plasma, contain sufiicient momentum to bombard thelithium niobate cathode and remove small particles oflithium niobate for subsequent deposition on a substrate. This phenomenon is well known and is accomplished in known RF sputtering apparatuses.

Step 5.Presputtering to insure the cleanliness of the cathode with the shutter shielding the substrate.

Presputtering insures that the surface of target 12 is clean when actual deposition onto substrate 14 begins.

Presputtering may be carried out for times in the range of to 30 minutes with minutes being a typical time period. Most known RF sputtering techniques utilize a similar presputtering step for the same reasons. During the presputtering period, the hot pressed lithium niobate cathode 12 is subjected to radio frequency heating and may experience a considerable rise in temperature which, if sputtering were continued without interruption, would lead, on one hand, to a 'loss of oxygen from the lithium niobate cathode 12 resulting in non-stoichiometric single crystal films or, on the other hand, leading to spalling and ultimate breakage of the lithium niobate cathode due to overheating. It has been found that unless the lithium niobate cathode 12 is cooled for approximately 15 minutes after presputtering, that the resulting films, while single crystal, are non-stoichiometric. Non-stoichiometric single crystal films, in many instances, have characteristics which are undesirable and which make them less acceptable for use than stoichiometric single crystal films of lithium niobate. Thus, cooling after presputtering while not an essential step in providing single crystal thin films of lithium niobate represents one way of insuring the formation of single crystal thin films of lithium niobate which are stoichiometric. Another technique which insures the formation of stoichiometric single crystal thin films is to reduce the RF power density and thereby sputter at a lower deposition rate 04 A.) and achieving less heating of the. cathode than when higher RF powers are utilized.

Step"6.Applying radio frequency power to the lithium niobate cathode sufiicient to provide a power density of 2-12 watts/in. between the electrodes for a time sufficient to deposit a single crystal film of lithium niobate of required thickness on a substrate heated to a temperature in the range of 450 C.870 C.

After presputtering, shutter 38 is moved away from its position between target 12 and substrate 14. It should be recalled that the substrate has been heated to a temperature in the range of 450 C. to 870 C. prior to the presputtering step. When an RF potential is applied between target 12 and substrate 14' an induced substrate bias greater than the wall potential exists prior to the deposition of target atom onto the substrate. The RF potential is approximately 1,000 volts. The geometry of the system, the capacitance to ground of substrate 14 and the peak-to-peak voltage of the applied RF wave determine the magnitude of the induced substrate bias. Bias controls the alignment of electric diodes to get poling in as deposited single crystal LiNbO when prepared by RF sputtering techniques.

In connection with the deposition rate parameter, it has been noted that once the deposition rate exceeds 0.4 A. per second that regardless of the substrate temperature, argon pressure or power utilized, the resulting deposited films of lithium niobate are not single crystal. It has also been determined experimentally that the deposition rate increases up to a point with increasing pressure of the argon gas in the system. Thereafter, the deposition rate begins to decrease with increasing argon pressure when intra plasma collisions cause a reduction in the overall number of ionized particles effective in sputtering. In view of this, the highest deposition rates would be achieved at the high end of the 2544 microns range of argon gas pressures. Sputtering, however, is preferentially carried out at a fixed pressure of 32 microns.

In connection with the temperature range of 450- 870 C., it has been found that outside of these ranges, the resultingdeposited. films are at best oriented crystals of lithium niobate. Below 450 C., oriented films are obtained whereas above 870C, multiphase mixtures of component oxides are formed.

The deposition times are a function of the power utilized, the deposition rate and the desired thickness of the resulting film. As indicated hereinabove, films up to 10,000 A. in thickness and having surface areas which are limited only by the areas of available substrates and cathodes can be fabricated in accordance with the above method.

In connection with the power densities indicated above, when using a 4" diameter cathode, powers in the range of 25-100 watts are preferably used when single crystal stoichiometric thin films of lithium niobate are desired. However, where non-stoichiometric single crystal films of lithium niobate are suitable, powers of up to 150 watts may be utilized. Care should be taken, however, not to exceed the deposition rate of 0.4 A. per second since at higher deposition rates only polycrystalline films are obtained. All other things being equal, deposition rate increases with increasing power.

To more clearly define the conditions under which single crystal films of lithium niobate have been deposited by RF sputtering, the following examples are provided:

EXAMPLE I Lithium niobate was deposited on c-sapphire (ct-A1 0 by RF sputtering to form a single crystal thin film of lithium niobate. The sputtering conditions were:

Initial system pressure 2X10 torr; substrate temperature of 450 C.;

Pre-sputtering for 15 minutes at 32 microns argon pressure;

Cooling after pre-sputtering for approximately 30 minutes;

Power: watts on a 4" diameter cathode;

Sputtering from target to substrate at approximately 32 microns argon pressure;

Deposition rate: 0.4 A. per second.

Under the above sputtering conditions, a stoichiometric single crystal film of lithium niobate having a thickness of 3200 A. was deposited. The deposited film was determined to be single crystal by the well known Seeman- Bohlin technique.

In the Seeman-Bohlin diffraction arrangement, the specimen and focussing circle remain sationary while the detector tube moves along the circumference of the actual focussing circle itself. In this arrangement, a single crystal will not give rise to diffraction peaks, whereas polycrystalline would. Thus, Seeman-Bohlin technique could be used to screen single crystal films from polycrystalline ones. In characterizing thin films by the Seeman-Bohlin technique, a modification of the method developed by R. Feder and B. Berry was used. (IBM Technical Disclosure Bulletin, vol. 11, No. 12, May 1969, page 1728.)

EXAMPLE H The conditions are the same as in Example I except that the cooling step after pre-sputtering was eliminated.

Under these conditions, the resulting thin film of lithium niobate is single crystal but nonstoichiometric in nature.

EXAMPLE III The same conditions as Example I except that the power applied on a 4" diameter lithium niobate cathode is 150 watts.

Again, the resulting lithium niobate film is single crystal but nonstoichiome'tric in nature.

EXAMPLE IV The conditions are the same as in Example I except for the following:

Substrate temperature600 C. Powerwatts Deposition rate-0.3 A./sec.

The resulting film is single crystal but nonstoichiometric in character.

EXAMPLE v i The conditions are the same as in Example I except for the following:

Substrate temperature850 C. Power150 watts Deposition rate0.2 A./sec.

The resulting film is single crystal but nonstoichiometric in character.

Where the resulting single crystal of lithium niobate is nonstoichiometric in character, a post deposition anneal in one micron oxygen in an argon-oxygen ambient may be carried out to convert such film to stoichiometric lithium niobate film.

Typical annealing conditions for bulk crystals are: Heating a one-inch thick bulk crystal for approximately 10 hours at a temperature of l000 C.

Annealing conditions required for the RF sputtered single crystals of the present application are: Heating for 2 minutes at 400 C. or, for 5 minutes at 250 C., in the abovementioned argon-oxygen ambient.

From the foregoing, it can be seen that the abovedescribed method provides large area single crystals of lithium niobate which are uniformly reproducible and having excellent optical transmission characteristics. The technique produces extremely thin film of lithium niobate as opposed to those obtainable from grown crystals of the material. In addition, the quality and character istics of the resulting films are more controllable. For example, growth striations which occur in conventional crystal growth techniques appear as narrow bands of a second phase precipitate, exhibiting a definite symmetry and conforming to the shape interface. The presence of striae limits the usefulness of bulk lithium niobate single crystals for optical applications. Single crystal films of lithium niobate grown using the method of the present application are particularly significant since they do not have striae. The present technique is also relatively inexpensive when compared with conventional crystal growth techniques. The present technique is the only method of making adequate range on a single crystal LiNbO material.

While the invention has been particularly described with reference to specific examples thereof, it will be understood by those skilled in the art, that various changes in procedure may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method for depositing single crystal lithium niobate films by radio frequency sputtering comprising the steps of:

positioning a hot pressed cathode of lithium niobate in radio frequency sputtering apparatus,

heating a single crystal substrate in said radio frequency sputtering apparatus to a temperature in the range of -450-870 C., and

. sputtering in an argon atmosphere at a pressure in the range of 25 to 44 microns at a power density in the range of 212.5 watts/in. at a deposition rate of 0.4 A./second and less to deposit a single crystal film of lithium niobate.

2. A method according to claim 1 further including the step of:

pre-sp'uttering said cathode with said substrate shielded for up to 30 minutes and cooling said cathode prior to sputtering to prevent the finally deposited lithium niobate single crystal film from being non-stoichiometric in nature.

3. A method according to claim 1 wherein said single crystal substrate is a-cut sapphire.

4. A method according to claim 1 wherein said single crystal substrate is cut sapphire.

5. A method according to claim 1 wherein said single crystal substrate is a grown single crystal of lithium niobate. t

6. A method according to claim 1 further including the step of annealing said deposited film in an argon oxygen ambient for a time sufiicient to convert said single crystal film to a stoichiometric single crystal film.

7. A method according to claim 6 wherein said argon is present in said argon-oxygen ambient at a pressure of 1 micron.

8. A method according to claim 6 wherein said annealing step is carried out at a temperature of 600 C. and

below.

9. A method according to claim 2 wherein said cooling step is carried out for times of 30 minutes and less.

10. A method for depositing single crystal lithium niobate films by radio frequency sputtering comprising the steps of:

positioning a hot pressed cathode of lithium niobate ina radio frequency sputtering apparatus, heating a single crystal substrate of c-cut. sapphire in said apparatus to a temperature of 450 C., and sputtering in an argon atmosphere at apressure of. 32 microns at a power density of 8 watts/in. and at a deposition rate of 0.4 A./second-to deposit a single crystal film of lithium niobate.

11. A method according to claim 10 further including the step of:

pre-sputtering said cathode with said'substrate shielded for 15 minutes and cooling said= cathode prior to sputtering to prevent the finally deposited lithium niobate film from being non-stoichiometric in nature.

12. A method according to claim 10 furtherincluding the step of annealing said deposited-film in an argonoxygen ambient containing oxygen at a pressure of 1 micron and at a temperature of 400'? C. for 2 minutes to improve oxygen stoichiometry.

13. A method for depositing single crystal lithium niobate films by radio frequency sputtering comprising the steps of: a

positioning a hot pressed cathode of lithium niobate in radio frequency sputtering apparatus, heating a single crystal substrate in said apparatus to a temperature in the range of 450 C.870' (3., and sputtering in an argon atmosphere at a pressure in the range of 25-44 microns at a power density of watts/in. at a deposition rate of 0.4 A./secon'd and less to deposit a single crystal film of stoichiometric lithium niobate.

14. A method according to claim 13 wherein said single crystal substrate is one selected from .thegroup COHSlSlI'. ing of a-cut sapphire, c-cut sapphire and a grown single crystal of lithium niobate. g

15. A method for depositing single crystal lithium niobate films by radio frequency sputtering comprising the steps of; p

introducing a single crystal substrate selected from the group consisting of grown single crystal lithium niobate, .a-cut sapphire and c-cut sapphire'intoradio frequency sputtering apparatus, positioning a hot pressed cathode of lithium niobate. parallel to and spaced from said-substrate in said apparatus, I evacuating said apparatus to a pressure of approxi- 'mately 2x10 torr, i I P gettering undesired gases from said apparatus, introducing argon into said apparatus at'a pressure of 450-870 0., V i 7 g pre-sputtering with a'shieldecl substrate to insure'cleanliness of said cathode for up to 30 minutes, and I sputtering in an argon atmosphere at a pressure in the range of 25 to 44 microns at a power density in the range of 212.5 watts/in. at a deposition rate of 0.4 A./ second and less to deposit a single crystal film of lithium niobate.

16. A method according to claim 15 further including the step of:

cooling said cathode prior to said sputtering step to prevent the finally deposited film from being nonstoichiometric in nature.

17. A method according to claim 15 further including the step of annealing the deposited film in an argonoxygen ambient wherein orygen is present at a pressure of 1 micron at a temperature of 870 C. and below to improve oxygen stoichiometry, said time of heating decreasing with increasing temperature.

References Cited UNITED STATES PATENTS OTHER REFERENCES Poster, The Deposition and Piezoelectric Characteristics of Sputtered Lithium Niobate Films, J. of App.

0 Phys, vol. 40, 1969.

JOHN H. MACK, Primary Examiner S S. KANTER, Assistant Examiner 

